1. Field of the Invention
The present invention relates to zinc-aluminum coatings on a steel substrate.
2. Prior Art
Zinc-aluminum alloys have long been investigated as corrosion resistant coatings for iron and steel. Principally, a Zn-5% Al alloy and a 55% Al alloy coatings have been commercialized. The Zn-5% Al alloy is offered in two variations of the basic composition, one with additions of mischmetal to high purity zinc and the other with small additions of magnesium to a lower grade of zinc to compensate for the impurities in the zinc.
U.S. Pat. No. 4,448,748 to myself and others relates to the low aluminum content alloy containing from about 85% to about 97% zinc, from about 3% to about 15% aluminum and from about 5 ppm to about 1.0% mischmetal. The alloy may also contain one or more of the elements Fe, Pb, Sb, Mg, Sn, Cu and Si. This patent in comparing the mischmetal-containing Zn-5% Al coating to the high aluminum coating, Zn-55Al-1.5 Si, in column 4, lines 56-59, sets forth the superiority of the low aluminum mischmetal alloy.
The Zn-55% Al alloy employed commercially is nominally 55% Al, 43.5% Zn and 1.5% Si. Protective zinc-aluminum surface coatings containing relatively large amounts of aluminum were developed as set forth in U.S. Pat. Nos. 3,343,930 and 3,393,089 to take advantage of the improved oxidation resistance of aluminum and oxidation resistance under certain environments when compared with conventional lower aluminum content alloys. Silicon additions are required to these compositions to reduce the reactivity between the basic Zn-55% Al alloy and the steel substance during the coating operation when preheated steel sheet or wire is passed through the molten zinc-aluminum alloy bath. In spite of improvements exhibited by these higher aluminum content alloys, difficulties have been encountered in their production and with their use. The desirable favorable properties can not readily be achieved in practice.
In continuous galvanizing processes which include a pretreatment operation in a reducing atmosphere of hydrogen, the use of aluminum at a concentration above about 10% in the coating bath tends to create serious operating difficulties because of problems with these zinc-aluminum alloy baths. During the coating operation, it is difficult to achieve complete coverage of the substrate and bare spots occur. These sites or bare spots are foci for failure of the coating penetration of a corrosive medium. A large number of bare spots can cause premature corrosion of the composite coated steel.
Other factors contribute to the greater corrosion tendencies of the Zn-55% Al alloy. The higher aluminum alloy has a tendency to form a coarse grained structure composed of an aluminum-rich phase, a zinc-rich phase and particles of silicon. The large grain structure limits the thickness of the coating which can be applied due to the inherent instability of large grains as a coating. Less protection is afforded by thin coatings which can pit causing premature rusting.
The characteristic grain structure of the Zn-55% Al alloy sets up a series of electrolytic cells of anodes and cathodes formed by the varying chemistry of the grains in the coating. The silicon behaves in a manner cathodic to the zinc-rich and aluminum-rich phases and the aluminum-rich phase behaves in a manner cathodic to the zinc-rich phase. As a result, the zinc-rich phase in a corrosive medium cathodically protects the aluminum-rich phase and corrodes yielding a pitting type corrosion.
These electrolytic cells are a basic problem with the aforementioned alloy system. It causes a further problem with material that is painted, particularly material that is painted on a continuous coil coating line. At the edges of the painted sheet where it is cut, corrosion occurs. This corrosion is further accentuated by the anode-cathode behavior of the two major components of the metallic coating; namely, the zinc-rich phase versus the aluminum-rich phase. As corrosion products are formed by the action of the zinc-rich phase protecting the aluminum-rich phase, the oxidation products lift the paint film away from the metallic coating. As the film is lifted, a new corrosion mechanism; namely, differential aeration cell formation takes place accelerating the lifting of the paint film causing unsightly areas. This failure of painted material is called edge creep and is a serious weakness of zinc-55% aluminum alloy.
Additional problems with coatings in the higher aluminum range have been recognized in the prior art. Higher aluminum content coatings have limited ductility. The smaller the radii of bends made in the coated steel, the greater the degree of cracking in the coating. On the bases of these facts, it is recognized, as set forth in U.S. Pat. No. 4,152,472 to Ohbu et al, in column 3, that the baths containing lower aluminum concentrations, on the order of from 3% to 10%, are preferably used since adherent coatings are more easily achieved.
Prior art attempts to deal with the problems of the higher aluminum alloys concentrate on surface or substrate modification. Note that U.S. Pat. No. 4,330,598 to Lee et al requires the use of a low alloy low-carbon steel strip containing between about 0.03 to 0.1% carbon having titanium as an essential alloying element in the substrate.